Waste paper, commonly referred to as secondary fiber, is often used in the manufacture of paper and paperboard products. At present, the paperboard industry is the primary user of the recovered secondary fiber, and is moving toward the use of liner and corrugated media composed of all secondary fiber.
The secondary fiber frequently contains asphaltic materials, as well as waxes, and if the asphalt and waxes are not fully dispersed, they will appear as surface deposits which can result in a build-up on the press rolls of the paper machine and downgrade the finished board.
In a typical secondary fiber pulping system, the waste paper is introduced into a pulper and agitated with water to provide a stock having a solids content in the range of about 1% to 5%. After filtering through centrifuge cleaners and screens, the stock has traditionally been passed through an asphalt dispersion system in which the stock is thickened to a consistency of about 10% to 12% and then fed through a press where additional water is removed to raise the solids content to a range of 30% to 35%. The thickened stock is then introduced a into pre-heater where the stock is agitated and heated to a temperature of 250.degree. F. to 325.degree. F. to melt the asphalt and waxes. The stock is then blown into a cyclone or blowtank. At this point, water is added from the paperboard machine in order to decrease the solids content to a value of 4% to 5%. From the cyclone the stock is flowed to a stock chest and is pumped to the refiners immediately ahead of the papermaking machine.
In the conventional asphalt dispersion system substantial capital equipment has been required to thicken the stock to a solids content of about 30% to 35%, heat and defibrate the stock, and then return the stock to its approximate original consistency. Not only did the conventional system require substantial capital investment for equipment, but the energy costs were extremely high and considerable floor space was required for the equipment.
A variety of steam contact devices have been used in the past for heating liquids. Basic to all steam contact devices is the introduction of steam into the cold liquid utilizing a number of different device configurations, none of which are capable of heating a fibrous stock defibrating the fibers, and dispersing the contaminants with a single piece of equipment.